The present invention is directed to semiconductor devices and, more particularly, to semiconductor devices which incorporate a metal-to-semiconductor rectifying junction, such as Schottky diodes.
Semiconductor devices having a metal-to-semiconductor rectifying junction, known as a Schottky junction, are desirable for many applications. As an example, diodes having such a junction, known as Schottky diodes, are desired for applications where energy losses while switching from forward bias to reverse bias and back can significantly impact the efficiency of a system and where high current conduction is desired under forward bias and little or no conduction is desired under reverse bias, such as when used as an output rectifier in a switching power supply. The Schottky diodes have lower turn-on voltages because of the lower barrier height of the rectifying metal-to-semiconductor junction and have faster switching speeds because they are primarily majority carrier devices.
The rectifying properties of the metal-to-doped semiconductor junction results from a contact potential difference that is based on the difference in the respective work functions of the contacting metallic layer and semiconductor body as well as on the semiconductor surface states. When the device is forward biased, the contact potential difference is overcome and the device conducts. By contrast, when the device is reverse biased, little or no current flows in the reverse direction. However, the applied reverse bias voltage can create high electric fields in the semiconductor body regions near the edge of the metallic contact, resulting in leakage current. Further, when the reverse bias voltage is sufficiently high, the high field concentration increases and may result in avalanche breakdown in the device.
To reduce the edge effects of the reverse biased Schottky contact, a guard ring may be formed in the semiconductor body in the region beneath the edge of the metallic contact. The guard ring is typically a region of conductivity type opposite to that of the semiconductor body which reduces the electric field near the edge of the metallic contact and reduces the radius of curvature effects at these edges. As a result, the breakdown voltage of the Schottky barrier is raised and the leakage characteristics are improved so that the device is suitable for higher-voltage applications.
Typically, the guard ring is formed by ion implanting a dopant species into the semiconductor body where the dopant species is of opposite conductivity type to that of the semiconductor body. To incorporate the dopants into the semiconductor body, a high temperature anneal is carried out which causes implanted dopant atoms to be substituted for atoms of the semiconductor body at some of the locations in the crystal lattice. When the implanted dose is sufficiently high, the anneal causes the conductivity type of the implanted regions to be reversed. For some semiconductor materials such as nitride-based semiconductors, however, such a reversal is hard to achieve. Further, the annealing process damages the surface of the semiconductor body- and results in a poor contact between the metallic contact layer and the semiconductor.
It is therefore desirable to provide a guard ring for a semiconductor device having a Schottky junction without damaging the surface of the semiconductor.